A study by Wang et al in the BMJ (340:c1344) found that 21 out of 90 papers (23%) reporting studies on the antidiabetic agent rosiglitazone had incomplete Conflict of Interest information and 3 of the 21 stated that the authors had no competing interests although their other publications indicated that they did.

It's possible that this may, partly, be explained by journals using different thresholds for CoIs, but it suggests that editors may not be requesting or checking CoI statements sufficiently thoroughly.